Note concerning the coding of the form factors FF
       
In the coding of the different form factors, we have taken the 
following convention:

       3D : V(q) = e^2 / (EPS_0 * q^2)  * FF
       
       2D : V(q) = e^2 / (2*EPS_0 * q)  * FF
       
       1D : V(q) = e^2 / EPS_0          * FF